It is of great importance to minimise the size and weight of motor drive systems for mobile applications. Such motor drive systems generally derive electrical power for their operation from a mechanical source that comprises a prime mover, such as an engine. An electrical generator converts mechanical power from the prime mover into electrical power. Most commonly, the motor drive system utilises existing AC power and converts it variable frequency AC to drive a multiphase AC motor. The multiphase AC has a frequency or range of frequencies, such as zero to approximately 400 Hz, which is sufficient to drive the AC motor through a desired range of speeds.
The electrical generator generally has a multiphase AC output at a relatively low fixed fundamental frequency, generally about 50, 60, or 400 Hz, or a variable frequency that covers the range of approximately 360 to 800 Hz, which requires conversion in the motor drive system from generator multiphase AC output to the range of frequencies sufficient to drive the AC motor at the desired range of speeds. Such a motor drive system requires high power electronics, which add size, weight and cost to the system. It would be desirable to use low power electronics to vary the output of the generator itself to produce the necessary range of frequencies sufficient to drive the AC motor at the desired range of speeds.